Known in the art are several methods for preparing isocyanates.
One of those methods comprises interaction of alkyl ethers of sulphuric or phosphoric acid with cyanates of metals. The yield of isocyanates is as high as 90% by weight. However, the starting ethers are rarely available and, in some cases, toxic compounds.
Exchange-type interaction of haloalkyls with silver cyanate is of interest only under laboratory conditions due to silver shortage. The method based on the addition of isocyanic acid to unsaturated compounds makes it possible to prepare only lower monoisocyanates.
The synthesis of monoisocyanates by carbonylation of amines is ineffective due to the use of equimolar amounts of rather expensive palladium chloride (E. Stern, M. L. Spector J. Org. Chem. 31,596, 1966).
Preparation of isocyanates by way of catalytic carbonylation of nitroalkyls requires high temperature within the range of from 100.degree. to 250.degree. C. and pressures of from 100 to 500 atm. The method of preparing isocyanates (with a yield of from 42 to 73%) by decomposition of N-formamides in the presence of chlorine-containing agents has not obtained practical application due to the multi-stage character of the process.
The classic method of preparing isocyanates by phosgenation of amines is complicated, since it necessitates the use of high temperatures and dehydrochlorinating agents. Phosgenation of alkylaminosilanes, in addition to the two-stage production process, necessitates the use of toxical phosgene.
Among phosgene-free methods the Kurcius method is a laboratory method due to the risk of explosion upon heating of inorganic and organic azides.
Also known in the art is preparation of a monoisocyanates by pyrolysis of esters of carbamic acid in the presence of P.sub.2 O.sub.5 at a high temperature (100.degree. to 500.degree. C.), but due to the inversible character of the process, the product is obtained with a low yield.
Therefore, at the present time in the art there is a need in a suitable, versatile method for the preparation of said organic compounds.